The present invention pertains in general to methods and apparatus for measuring properties of samples exposed to radioactive sources and in particular to methods and apparatus for measuring the thermal neutron absorption cross-section for rock and fluid formation samples.
Pulsed-neutron logging is one of the well-logging techniques applied in order to provide information relating to the content of hydrocarbons in subsurface geological formations. Pulsed-neutron techniques are commonly used to determine the neutron life time of a formation such that EQU .tau.=1/(v.SIGMA..sub.a) (1)
where:
.tau.=the neutron life time of the formation; PA1 v=the thermal neutron velocity in the formation; and PA1 .SIGMA..sub.a =the absorption cross-section of thermal neutrons in the formation. PA1 .phi.=the porosity of the formation; PA1 V.sub.sh =the shale content of the formation; PA1 .SIGMA..sub.r =the absorption cross-section of the formation's rock matrix; PA1 .SIGMA..sub.sh =the absorption cross-section of the shale in the formation; PA1 .SIGMA..sub.h =the absorption cross-section of formation hydrocarbons; PA1 .SIGMA..sub.w =the absorption cross-section of formation water; and where PA1 .SIGMA..sub.a is as defined above.
As can be seen from Equation (1), the thermal neutron absorption cross-section of formation materials may be deduced, assuming that the thermal neutron velocity .nu. is known. However, the thermal neutron absorption cross-section of the formation .SIGMA..sub.a does not by itself provide an indication of hydrocarbon composition because it is a linear combination of the thermal neutron absorption cross-sections of all rock components. For example, in a particular well, EQU .SIGMA..sub.a =(1-.phi.-V.sub.sh).SIGMA..sub.r +V.sub.sh .SIGMA..sub.sh +.phi.(1-S.sub.w).SIGMA..sub.h +.phi.S.sub.w .SIGMA..sub.w ( 2)
where:
Consequently, it is useful to be able to determine the thermal neutron absorption cross-sections of samples of formation materials in order to be better able to interpret the results of pulsed neutron well logging.
Most methods for measuring thermal neutron absorption cross-sections have required the use of samples which are relatively large in mass or volume. Methods which permit relatively small sample sizes tend to have relatively large errors.
Some techniques have been useful only with fluids, while others have been useful only for rock samples. Some of the techniques which apply to formation fluids, require dilution of those fluids to make up a required volume of up to one gallon.
Measurements of thermal neutron absorption cross-section are obtained from pulsed neutron die-away measurement or steady-state thermal neutron flux measurements. Existing approaches employing die-away measurements rely upon ractors and accelerators as sources of neutrons. Because such techniques require the use of complicated and costly equipment, they tend to be done at central locations far from the source of the material and are both costly and time consuming to obtain. Flux measurements are made using steady-state sources which are simple and inexpensive to use. However, the relative positions of the neutron source, the sample and the detector, are very important in determining the sensitivity of a flux measurement.
Because accelerators and reactors are large pieces of equipment, they do not readily provide small neutron sources capable of placement within a sample. Consequently, the source is positioned at some distance away from the sample. A detector which is particularly sensitive to thermal as oppossed to fast neutrons, is placed within the sample or around the periphery of the sample but not between the sample and source. Losses due to scattering of neutrons create problems of calculation and interpretation.
Although thermal neutron flux measurement may be made by using the same configurations of sample, source and detector required by pulsed neutrons measurements, a small source and the reduced degree of scattering allow different configurations to be implemented. Specifically, the source has been mounted parallel to or concentrically on the detector within a fluid sample in the absence of a moderator. However, such techniques have required dilution up to a volume of one gallon, making them impractical for small samples in general, and solid samples in particular.